1. Field of the Invention
This invention relates to a method of making a thermal management circuit board or a thermal management circuit board panel with printed circuits formed thereon.
2. Prior Art Statement
Many components used in electronic assemblies generate large amounts of heat during operation. Such heat, if not removed, can cause premature failure of the component, the circuit board it is mounted on and/or the surrounding electronic devices. In extreme cases, if the heat coming from the electronic device is not dissipated or removed from the mounting area, component fire and/or entire system failure could occur. A study of the prior art shows that presently available constructions employ such methods as:
1. Heat dissipating metal heat sinks applied to the component.
2. Circuit boards utilizing thin thermally conductive dielectric materials attached to a metal substrate.
3. Areas milled in a circuit board exposing an underlying metal substrate that a component can rest upon.
While able to dissipate the heat of the component, or circuit to some extent, issues are still present in the heat dissipation, as the component is not directly attached to an underlying metal core (1 and 2 above), even though the core is often attached to supports for heat reduction.
In cases where the dielectric area is removed and the component is placed on the core (#3 above), surface mount devices cannot be used because the leads must be on the same plane as the bottom on the device. Additionally these types of boards often require additional layers for electrical interconnections. With high heat generating electronic components becoming commonplace in electronic circuits thus requiring management of the thermal characteristics, problems with the assembly of the electrical components to the circuit board also arise, especially with surface mount components. Miniaturized surface mount components often rely upon the circuit trace to dissipate heat from the component which is inefficient. In other surface mount devices, a heat sink is placed upon the exposed surface of the heat generating component requiring another costly step in the manufacture of the circuit. Thus, there is a need to provide a thermal management circuit board for surface mount components that has raised portions on the heat sink material registered with apertures through the electrically isolating material and the prepreg between the circuit traces such that the surface mount component is placed directly upon the raised portion of the heat sink therefore improving thermal management of the circuit board.
It is known to provide a printed circuit board comprising a laminated metal substrate designed for dissipating heat having a curable dielectric material located on the substrate, wells in the dielectric material contacting the substrate created by masking the curable dielectric material at the location of the wells, the wells further having active circuit components mounted therein which contact the substrate, the laminated metal substrate having a coefficient of thermal expansion which matches that of the active circuit components. For instance, see the U.S. Pat. No. 4,999,740 issued on Mar. 12, 1991 to Ilardi, et al. Thus, though thermal management of a circuit is achieved by removing the dielectric material in the area of heat generating circuit components, it has been found that this is impossible with surface mount circuit boards as described above where the surface mount component must be re-flow soldered to the circuit trace on the same plane as the bottom of the component. Thus, the need to have surface mount components mounted directly upon raised portions of the heat sink is desirable thus placing the bottom of the component directly on the heat sink while allowing re-flow soldering of the component to the trace.
It is also known to provide a method of preparing a circuit board on a dielectric and then applying the circuit to the circuit board with a heat conductive medium placed between the circuit board and the component. A silicon grease is coated on the circuit board side of the heat producing element and a projecting portion of a heat sink is inserted through a through hole under the heat producing element. The projecting portion and the circuit board are placed in contact with the heat sink and the circuit board is affixed to the heat sink with Mylar® tape. Contact with the heat producing element is through the silicon grease. Excessive silicon grease is collected in a step portion provided into the heat sink in an area surrounding a projection portion contact platform. The bottom surface of the circuit board and the bottom surface of the projection portion are on the same plane, that plane coincident with the top of the heat sink. For instance, see the U.S. Pat. No. 6,297,959 B1 issued on Oct. 2, 2001 to Ueno, et al. The multiple steps in this process makes manufacture of circuit boards cost prohibitive. In addition, it is impossible to secure miniaturized circuit components with Mylar® tape and equally impossible to secure a heat sink to a circuit board having a plurality of miniaturized circuit components surface mounted thereupon. Therefore, there is a great need for a method of directly mounting surface mount components to a heat sink in the process of securing the component to a circuit board.
In addition, it is known to provide a heat sink which has a “button” on its inner surface extending through a circuit board. The button is affixed to the heat generating semiconductor with a thermally conductive adhesive device and relies upon the thermally conductive adhesive to transfer heat away from the device to the heat sink. For instance, see the U.S. Pat. No. 5,287,247 issued on 15 Feb. 1994 to Smits, et al. Mechanical registry of a single button with a single heat generating semiconductor is possible, however, when multiple high heat generating components are to be mounted to a single board within a confined space, mechanical registry of the heat sink with the heat generating components is impossible. Thus, there is a strong desire to provide a thermal management circuit board with raised portions of a heat sink disposed on the same plane as the upper surface of the circuit trace to enhance mounting of multiple components to the heat sink while attaching the leads of the components to the circuit trace.
It is further known to provide a method of manufacturing an electrically driven LED lamp assembly comprising the steps of disposing an electrically insulating coating of less than 1000 microns thickness over an electrically and thermally conductive, entirely metal heat sink, printing circuit traces on the coating and adhesively securing light emitting diodes to the circuit traces with an electrically and thermally conductive adhesive. For instance, see U.S. Pat. No. 5,857,767 issued on Jan. 12, 1999 to Peter A. Hochstein. Hochstein provides thermal management at circuit assembly by avoiding the soldering of components to the circuit board, however it has been found that the teachings of Hochstein cannot be applied to surface mount components. Thus, there is still a great need for a method of mounting surface mount components directly upon raised portions of a heat sink with the circuit trace on the same plane as the raised portion of the heat sink.
Also known is to provide a circuit board panel comprising a copper or aluminum substrate, a layer of epoxy and a layer of copper wherein the layer of epoxy contains finely comminuted, thermally conductive, electrically insulating material such as a ceramic material. For instance, see the technical bulletin entitled Thermal Clad© Overview available from The Bergquist Company, 18930 W. 78th Street, Chanhassen, Minn. 55317 USA. Though thermal management of a circuit board is enhanced by providing thermally conductive material in the layer of epoxy, direct contact with the heat sink has not been taught and therefore there is still a need for a circuit board having a means for mounting surface mount components in direct contact with raised portions of the heat sink to allow for simultaneous mounting of the leads of a heat generating component and the base thereof to the heat sink.
Finally, it is known to mount a heat producing light emitting diode (LED) in intimate contact with a projection integrally formed on a heat sink with the top of the heat sink projection on the same plane as the top of the circuit board traces wherein an insulating member is provided with a through hole exactly the same diameter as the projection and is adhered to the heat sink with an adhesive. The insulating member is additionally formed with cup-shaped cavities which direct the light emitting from the LED's. For instance, see the U.S. Pat. No. 6,874,910 B2 issued on Apr. 5, 2005 to Sugimoto, et al. Precision forming by mechanical processes is cost prohibitive for most circuit board manufacture as is precision forming of insulating members. Thus, the need for a cost effective and simple procedure for producing thermal management circuit boards is greatly needed.